This invention generally relates to ventable seals for pressurized containers and, more particularly, to ventable seals for electrochemical cells.
Electrochemical cells, such as cylindrical alkaline electrochemical cells, employ two electrochemically active materials and an aqueous electrolyte. The electrochemically active materials are typically manganese dioxide and zinc. These materials are conventionally housed in a cylindrical elongated container that is open on one end so that the electrochemically active materials and electrolyte can be inserted therein during the cell manufacturing process. A closure assembly that incorporates a disc shaped elastomeric seal body and an elongated metallic current collector that projects through the center of the seal body closes the open end of the container. The seal body usually includes a hub, which surrounds the collector, and a thin diaphragm integrally molded into the central region of the seal body. The function of the diaphragm is to rupture and release gas from within the cell when the internal pressure becomes too high. The collector provides a conductive path between the zinc and one of the cell""s terminal covers which is located on the end of the cell.
Manufacturers of electrochemical batteries constantly strive to improve the performance of their products in a wide variety of battery powered devices. While most batteries are used in a conventional manner, a small percentage of batteries are exposed to extreme or abuse conditions. One of the abuse conditions occurs when a battery experiences a direct electrical short. This condition occurs when a low resistance electrical path is established between the anode and cathode. In one scenario, a direct electrical short can occur when a contact spring in a device, such as a flashlight containing two D-size batteries, inadvertently bridges the gap between the edge of the battery""s steel container which contacts the cathode and the negative terminal cover that electrically contacts the anode. The spring is made of a highly conductive material such as nickel plated steel and thus provides a low resistance electrical connection between the anode and cathode. As soon as the direct electrical short is established, the cell begins to discharge as quickly as possible. In D-size batteries, which measure approximately 61 mm high and 34 mm in diameter, currents in excess of 20 amps are possible. Due to the exothermic chemical reactions that take place within a cell during the rapid discharge, the entire battery may reach temperatures in excess of 70xc2x0 C. The increase in temperature increases the pressure within the cell. In addition to increasing the temperature of the battery, the chemical reactions that take place during discharge rapidly generate quantities of hydrogen gas that substantially increase pressure within the cell. The simultaneous production of hydrogen gas and increase in temperature causes the elastomeric seal, which is typically made of nylon, to become soft and lose some of its structural rigidity. The thin ventable portion of the seal may become elongated due to both the heating of the nylon and the increase in internal pressure. Consequently, when the softened and distorted seal ruptures in response to the pressure buildup, an initial quantity of gas may escape from within the cell but the tear in the ruptured seal could be resealed when the softened ruptured seal contacts the smooth outer surface of the seal""s hub and reseals against the hub. If the ruptured seal does reseal against the hub and the cell continues to generate gas, the cell may eventually experience a crimp release wherein the crimped connection between the seal and container is broken and the closure assembly is forcefully ejected from the container.
As disclosed in U.S. Pat. No. 6,270,919 B1, previous attempts to prevent resealing of a ruptured seal body have included modifying a seal""s inner disc portion to include ribs. The ribs are designed to maintain the opening in a ruptured seal body thereby preventing resealing of the vent mechanism. However, while the inclusion of ribs in the seal""s diaphragm is helpful in preventing resealing in most cells, some cells with the ribs incorporated therein may be deformed when exposed to the heat generated during a direct electrical short such that the ribs cannot maintain the opening in the seal after it has ruptured.
In a seal design disclosed in U.S. Pat. No. 6,312,850 B1, vertical grooves were placed in the surface of a compression member that forms a part of the seal assembly. The grooves are designed to prevent resealing of a vented seal""s diaphragm. The grooves create channels that allow the gas to vent and thereby prevent resealing of the vented diaphragm. While this embodiment does prevent resealing of a vented seal, the compression member is an extra part that must be manufactured and assembled onto the seal body. This increases the cost of the battery and complicates the cell manufacturing process. Furthermore, the compression member occupies volume within the cell that would be better used to house electrochemically active materials.
Therefore, there exist a need for an inexpensive and simple to manufacture elastomeric seal body that occupies a minimum amount of volume within the cell and can reliably prevent resealing of a vented electrochemical cell.
The present invention provides a ventable seal body that prevents resealing of a ruptured seal in a pressurized container. The disc shaped seal body is manufactured as a single component including a first surface, a second surface, a flexible diaphragm formed between the surfaces and a protrusion that projects from the first surface. The protrusion includes a proximal section and a venting section. The proximal section, which abuts the flexible diaphragm at a ventable interface, includes an outer surface free of one or more indentations. The venting section, which is concentrically aligned with and abuts the proximal section, includes an outer surface with at least one indentation formed therein. The indentation in the venting section creates an unobstructed path along the outer surface of the venting section.
The present invention also provides for an electrochemical cell having a container with an open end, a closed end and a sidewall therebetween. The container includes a separator and two electrochemically active materials arranged on opposite sides of the separator. A disc shaped seal body formed as a single component is secured to the open end of the container. The seal body has a top surface, a bottom surface and a perimeter that contacts the top and bottom surfaces. A flexible diaphragm is formed between the surfaces and positioned around a centrally located protrusion that projects perpendicularly from the center of the seal body""s top surface. The protrusion defines an opening between the top and bottom surfaces. The protrusion has a proximal section abutting the flexible diaphragm at a ventable interface and a venting section concentrically aligned with and abutting the proximal section. The proximal section comprises an outer surface that is free of one or more indentations. The venting section has an outer surface with at least one indentation formed therein. The venting section""s indentation creates an unobstructed pressure relief path along the outer surface of the venting section. A current collector extends through the opening in the protrusion and contacts an electrochemically active material in the cell. A cover is positioned between the flexible diaphragm and the cell""s external environment.